The present invention relates to integrated circuits, and more particularly to substrates having through vias possibly containing conductive features.
Through vias with conductive features are used to shorten conductive paths between circuit elements in integrated-circuit packages. For example, FIG. 1 illustrates integrated circuit dies 110 attached to a printed circuit board (PCB) 120 through an interposer 130. The interposer includes a substrate 140 with metalized through vias 144. Compared to a direct attachment of dies 110 to PCB 120, the interposer can redistribute the contact pads to reduce the package area (the area of the entire structure). More particularly, dies 110 have contact pads 110C attached to the interposer's contact pads 130C.1 by solder features 150. The interposer has contact pads 130C.2 attached to contact pads 120C of PCB 120 with other solder features 150. Many fabrication processes allow smaller critical dimensions in dies 110 than in PCB 120. Therefore, the die contacts 110C can be smaller, and spaced closer to each other, than possible for PCB contacts 120C. Interposer 130 includes redistribution (rerouting) layers 154 with conductive lines 158 connecting the interposer contacts 110C.1 to the metal in vias 144. Lines 158 allow the PCB contacts to be redistributed. For example, if a die's contacts 110C are positioned on the die's periphery rather than being evenly distributed over the die's area, the corresponding PCB contacts 120C can be evenly distributed over an area equal to the die's area. Therefore, the spacing between the PCB contacts can be enlarged without increasing the area. Further, some contacts 110C on the same or different dies 110 may be designed for connection to the same input, e.g. the same signal or a power or ground voltage. Such contacts 110C may be connected to a single PCB contact 120C through lines 158, allowing the PCB contacts 120C to be fewer and occupy a smaller area. Thus, the area required for the die attachment is reduced.
Vias 144 should be narrow to reduce the package size. At the same time, the interposer's substrate 140 should be sufficiently thick to withstand the mechanical and electrical stresses and meet the heat distribution requirements during fabrication and operation. These two goals—narrow vias and a thick substrate—drive up the vias' aspect ratio. The high aspect ratio complicates both via formation and via filling with metal. In particular, it is difficult to provide reliable metallization, without voids or breaks, in high-aspect-ratio vias. Hence, the vias are widened to undesirably increase the package area.